1. Technical Field
The present invention relates to a method of strengthening a specified part of an aluminum casting such as a cylinder head of an engine.
2. Background Art
Cylinder heads of recent automotive engines bear greater thermal load as well as mechanical load as the engine output has been raised. Referring to FIG. 7 of the accompanying drawings, such an increase in the loads sometimes results in thermal cracking in a bottom face of a cylinder head 1 between an intake port 3 and an exhaust port 4 or between these ports and a fuel injection nozzle port 5, i.e., an intervalve part 6, which is thinner than other portions. Therefore, the intervalve part 6 should be strengthened to prevent the cracking. In other words, the intervalve part 6 is a part to be reinforced by the present invention (referred to as the "object part" hereinafter).
Referring to FIG. 8, a hatched potion in a piston 7, namely, a part that surrounds a combustion chamber 8, is also a part that has to bear a particularly high load and the object part of the present invention.
Conventionally, the following methods are available to strengthen the object part:
1. The alloying method: The object part is added with one or more of suitable heat resisting elements (e.g., Cr, Mo, Cu, and Ni) in an appropriate amount so as to improve the high temperature strength.
2. The cast-over method: The object part is covered or cast over, with steel or heat resisting material.
3. The grain refinement method: The object part is added with Sr or Ti of an appropriate amount so as to refine its granular structure on solidification, which brings about improvement in the resistance to thermal cracking through improved tensile strength and ductility.
4. The hard alumite treatment method: A hard alumite film of 100 micrometers or so in thickness is formed on the surface of the object part so as to improve the thermal resistance.
5. The remelting method: The object part is melted once again using a high density energy, such as laser beam, electron beam and TIG arc, so as to refine its granular structure on rapid solidification, bringing about improved high temperature strength.
However, these methods are not without drawbacks, as follows:
1. The alloying method: This method is not only rather expensive, but is liable to degrade, contrary to the present invention, the productivity and the high temperature strength because the castability is generally deteriorated on alloying, causing shrinkages and other casting defects or micro-shrinkages to form.
2. The cast-over method: This method needs the alumunizing treatment to be conducted on the steel material to be cast over. In addition, a great deal of care has to be paid about oxidation prevention, rigid control of casting temperature and post-casting non-destructive testing, calling for a large increase in the labor cost.
3. The grain refinement method: The benefit of this method is as small as it is convenient to practice.
4. The alumite treatment method: The effect of improving the heat resistivity of this method is greater than the methods 1 and 3, but all parts other than the object part must be covered with a masking so as to protect them from forming alumite film at an expense of additional labor and degradation in productivity.
5. The remelting method: This method is easy to practice and the benefit can be quite large, but the strengthening obtainable is not sufficient to attain the level desired here.
To realize such a level of strengthening as required for an automotive engine part, a method that combines the methods 1 and 5, i.e., the one called the remelting alloying method, has been proposed. In this method, the object part is remelted with a plasma welding machine and is added with a heat resisting element, e.g., Ni, Cu, Cr, Mo, W and the likes, in the powdery form singly or in combination. Since this method is capable of forming an alloy overlay having a higher content of heat resisting element or elements than the simple remelting method is able to, the object part ends up with having its heat and werar resistances much improved.
Though quite effective, this method has following drawbacks:
(a) When added singly in the powdery form, the heat resisting element is not very soluble in the molten base metal of Al, so that cracks are apt to occur at the interface between the alloy overlay and the base metal;
(b) When added in combination, cracks are apt to occur also, now because of the difference in the rate of shrinkage existing among the portions of the overlay if mixing is not homogeneous; and
(c) When a plasma welder is used, the gas contained in blowholes of the Al metal is liberated on melting, ending up entrapped in the weld overlay, making its porosity high.